Ascorbic Acid

Treating the wine with ascorbic acid which will break the disulfide back down to sulfide and adding copper sulphate (CuSO4.5H2O) solution to remove the sulfide.

Testing before treatment is absolutely necessary because it is possible to confuse the off-odor for Brettanomyces, which has a barnyard odor and cannot be eliminated by treating it for mercaptans. See our post about H2S.

To test, it is necessary to first make two stock solutions:

One of copper sulphate which is done by dissolving 4.1 grams in a little water and bringing the volume up to one liter with distilled water. (Use 10 ml of this solution with 90 ml of distilled water to make 100 ml total for the lab test.)

One of ascorbic acid which is done by dissolving 10 grams in a little water and bringing the volume up to one liter with distilled water.

Next, put 100 ml of the suspect wine into three glasses. Use the first glass as the control. Put 5 drops of the diluted copper sulphate solution into glass number two and stir well. Into glass number three, put 5 drops of the ascorbic acid solution, stir well and, after a few minutes, add 5 drops of the copper sulphate solution and stir well. The following table illustrates the possible results.

If disulfide is not present, addition of the copper solution will help; if disulfide is present, both ascorbic acid and copper sulphate must be used. To determine the amount of the copper solution to use, set up a series of glasses with 100 mls of wine and add 0.05 ml, 0.1 ml, 1.5 ml, etc. of the solution. Check the smell of each glass and select the first one that no longer smells. The addition of the copper solution used is the equivalent in parts per million of copper sulphate addition. Thus 0.1 ml = 0.1 ppm. To treat a 19 litre carboy of wine with 0.1 ppm requires 0.1 ppm x 19 = 1.9 ml of the stock copper solution.

Prior to adding the copper solution, add about 25 ppm of ascorbic acid, or about 0.5 grams in a 19 litre carboy. Stir in well and wait at least one day before adding the copper solution.

Ascorbic acid in conjunction with copper sulphate works very well, but it is not instantaneous; it takes several days before the odor and taste disappear. Do not exceed the recommended dosage of copper sulphate or you may induce a copper haze which will be difficult to remove.

Remember, H2S (volatile) à mono-mercaptans (becoming bound) à poly-mercaptans (bound), so deal with the problem as soon as it is detected. This process is not discrete: that is, while H2S is present, it is likely that mono-mercaptans are forming; and poly-mercaptans may be forming before the H2S in its volatile form disappears. Research shows that mercaptan formation occurs within two days after the beginning of fermentation and is at its peak at about two months after which the poly-mercaptans become dominant. Since most winemakers barrel-age their wines for much longer periods, if H2S has been detected and removed in the early stages, constant checking for mercaptan odours is critical since the barrel is where the mercaptans are formed, and they will continue to develop in the bottle.